Assuming that the outside diameter of the rotary mandrel of a printing machine in the flexographic, offset or rotogravure printing field is concentric with the mandrel's axis of rotation, then as the rotational speed of the print sleeve that is mounted on that mandrel increases, maintenance of adequate print quality increasingly depends on maintaining a fixed and invariable radial distance between the outside diameter of the rotary mandrel and the inside diameter of the print sleeve. If this radial distance varies, then print quality degrades. One type of degraded print quality takes the form of lightly inked or un-inked portions of the image alternating with darkly inked portions of the image. Another type of degraded print quality arises when portions of the image contain too much ink so as to decrease the desired resolution of that portion of the image on the substrate that advances past the printing surface of the print sleeve.
Variation in this desired fixed and invariable radial distance can occur if the print sleeve is subject to vibration as the print sleeve and the mandrel rotate. Such variation in the fixed and invariable radial distance can arise when an asymmetric printing surface of the print sleeve causes uneven pressure to be applied to the print sleeve, and this uneven pressure in turn causes a vibrational resonance effect to be transmitted to the bridge sleeve that results in the bridge sleeve becoming out of round as the print sleeve and the mandrel rotate. Such variation in the fixed and invariable radial distance can also occur for example due to the rotational inertia that acts on the bridge sleeve at very high run speeds and causes the bridge sleeve to become out-of-round as the print sleeve and the mandrel rotate.
In the flexographic, offset or rotogravure printing field, in order to increase the circumference of the printing surface without increasing the diameter of the rotary mandrel, it is known to use a bridge sleeve that is disposed between the outside cylindrical (or conical) surface of a rotary mandrel of the printing machine and the inside cylindrical (or conical) surface of an actual print sleeve, which carries on its outer cylindrical surface the data and/or images that are to be printed. The use of a bridge sleeve such as disclosed in commonly owned U.S. Pat. No. 5,782,181, which is hereby incorporated herein in its entirety for all purposes, enables various print developments to be achieved with the same rotary mandrel, without the need to replace this latter (generally of steel and hence heavy or of carbon fiber and hence costly) following a change in print development compared with the previous work carried out on the same printing machine.
However, a bridge sleeve that fails to serve as a rigid concentric attachment between the outside diameter of the rotary mandrel and the inside diameter of the print sleeve will fail to maintain a fixed and invariable radial distance between the outside diameter of the rotary mandrel and the inside diameter of the print sleeve and so result in the types of unsatisfactory print quality described above.
Various methods are known for mounting a conventional bridge sleeve (defined by a hollow cylinder with a through hole) onto a rotary mandrel of a printing machine. While mounting systems employing hydraulics and mounting systems employing mechanical connections are known, these typically are more cumbersome and heavier than a much used “air mounting” system that employs a conventional bridge sleeve that has an inner core layer, which though the inner core layer is slightly expandable in the radial direction, under atmospheric conditions the inner core layer defines an inner surface diameter slightly smaller than the diameter of the outer surface of the mandrel. The difference between these diameters enables an interference fit to be achieved between the mandrel of the printing machine and the conventional bridge sleeve. Positioning the conventional bridge sleeve at one end of the mandrel, compressed air is supplied (by known methods) between the outer surface of the mandrel and the inner surface of the bridge sleeve. The compressed air expands the diameter of the inner surface of the conventional bridge sleeve sufficiently to allow the bridge sleeve to slide over a cushion of air, a so-called air bearing, onto the outer surface of the mandrel. When the supply of compressed air is ended, the diameter of the inner surface of the conventional bridge sleeve shrinks sufficiently to allow the inner surface to grip the outer surface of the mandrel in an interference fit between the mandrel and the conventional bridge sleeve. Similarly, by again feeding compressed air onto the mandrel surface (by known methods), the inner surface of the conventional bridge sleeve can be slightly expanded to enable the conventional bridge sleeve to be released from the interference fit and removed from the mandrel.
Air-mountable bridge sleeves such as disclosed in commonly owned U.S. Pat. Nos. 5,819,657; 6,688,226; and 6,691,614, each of which being hereby incorporated herein in its entirety for all purposes, is usually made with a multi-layer body comprising a rigid outer cylinder made of carbon fiber and a cylindrical inner layer with an inner cylindrical surface that defines a bore with the diameter that is slightly smaller than the diameter of the outer surface of the mandrel. This type of conventional air-mounted bridge sleeve also includes at least one elastically compressible and radially deformable layer running the length of the bridge sleeve, and this compressible layer can be disposed against the outer cylindrical surface of the bridge sleeve's cylindrical inner layer. The compressed air acting against the inner surface of the inner layer of such a conventional bridge sleeve compresses this elastically compressible and radially deformable layer, which can be made of polyurethane foam for example, to enable the inner surface of the inner layer of the bridge sleeve to expand radially as it is being mounted on the outer surface of the mandrel.
However this elastic characteristic of the compressible layers of these air-mounted bridge sleeves works at cross purposes with the need for the bridge sleeve's outer surface to remain as rigidly fixed as possible with respect to the mandrel of the printing machine in order to resist the vibrations that are generated during operation of the modern printing machines that operate at very high run speeds. When the mandrel of such a printing machine rotates at speeds necessary to advance the substrate through the printing machine at line speeds of more than about 250 meters/minute, the non-uniform forces applied by the asymmetric printing surfaces of printing plates and/or the presence of the elastically compressible and radially deformable layer in a conventional bridge sleeve result(s) in machine vibrations that cause radial displacements of the bridge sleeve's outer surface with respect to the mandrel. These radially-directed displacements are transmitted to the printing surface of the print sleeve that is carried by the bridge sleeve, thereby causing the print sleeve to bounce against the substrate in rhythm with the vibrations instead of maintaining constant pressure contact with the substrate to be printed. The bouncing of the print sleeve against the substrate to be printed causes the printed image to include alternating regions where the image is printed darker than it should be followed by a region where the image is printed lighter than it should be printed. This bouncing also can cause some regions of the image to be too heavily inked and lose the desired resolution of the image. Accordingly, when these radial displacements of the bridge sleeve resulting from non-uniform pressures applied by the asymmetric surfaces of print sleeves and/or the deformation of the compressible layer do(es) arise, they compromise print quality to an unacceptable level by causing the type of banding or skipping described above to result from the bouncing of the print sleeve against the substrate.
These unacceptable radial displacements of the air-mounted bridge sleeve with compressible layers are more likely to arise as the sleeve's length and/or diameter increases. Nonetheless, printing machines that generate line speeds exceeding 250 meters/minute are becoming the norm, and a need exists for air-mountable bridge sleeves that produce acceptable print quality. Indeed, printing machines that generate line speeds exceeding 1,200 meters/minute are being put into service. Thus, as print line speeds increase and/or the diameters of the bridge sleeve must be increased in order to accommodate the larger print repeats that are needed to perform various print jobs, these air-mounted bridge sleeves requiring a lengthwise compressible layer fail to serve as a rigid concentric attachment between the outside diameter of the rotary mandrel and the inside diameter of the print sleeve.
Moreover, the elastically compressible and radially deformable layer running the length of the conventional bridge sleeve eventually degrades under even normal usage of a conventional bridge sleeve at lower line speeds below 250 meters/minute. Once this elastically compressible and radially deformable layer degrades, the entire bridge sleeve becomes useless and must be discarded, notwithstanding the continued viability of the remaining components such as the outer carbon fiber cylinder.
To eliminate the compressible layer (with its undesirable effects) of the air-mounted bridge sleeves, hydraulic systems have been developed for mounting bridge sleeves to the mandrel of a flexographic printing machine. One such hydraulic system for mounting a bridge sleeve on the rotary mandrel has been developed by Fischer & Krecke of Germany. This is an hydraulic system that requires a specially configured mandrel that has a smaller diameter on the operator side than on the motor side of the mandrel. The bridge sleeve has two end heads on which are mounted a carbon fiber cylinder. One end head defines a larger inner diameter that will fit over the larger diameter portion of the outer surface of the mandrel, and the other end head defines a smaller inner diameter that is nonetheless slightly larger than the smaller diameter portion of the outer surface of the mandrel at the operator end of the mandrel. At each end of the mandrel there is an expandable ring, the diameter of which expands and contracts according to the introduction or withdrawal of incompressible grease that is hydraulically used to expand or contract the rings. Each of these rings expands to contact the inner diameter of the steel insert at each end of a carbon fiber tube that forms the bridge sleeve.
Windmoeller Hoelscher of Germany has a mechanism that is similar to the Fischer & Krecke mechanism. The problem with each of these mechanisms is of course that as the rings expand and contract with usage, the rings become fatigued and their expansion eventually occurs non-uniformly so that they are not round relative to the central axis of the mandrel. Thus, over time the bridge sleeve rotates asymmetrically with the rotational axis of the mandrel, and this produces a bouncing motion of the bridge sleeve that causes the print quality to deteriorate as described above for the air-mounted bridge sleeves with the compressible layers. This deterioration is exacerbated as the speed of the web to be printed increases until the print quality is deemed unacceptable. Examples of unacceptable print quality include the presence of bands in the printed image that result from the bounce of the bridge sleeve as the rings that contact the inside diameter of the bridge sleeve no longer expand uniformly in perfect concentricity with the axis of rotation of the mandrel.
Another mechanical system for mounting a bridge sleeve on a rotary mandrel was developed by Paper Converting Machine Corporation of Green Bay, Wis. and is described in U.S. Pat. No. 6,647,879. In this PCMC system, the bridge sleeve has opposed hubs on which are mounted a carbon fiber cylinder. The internal diameter of each of these hubs is expanded and contracted by a semi-circular collar that has one end pivotally connected to its respective hub and the opposite end connected to its respective hub via an eccentric cam that opens and closes a pivoting clamp of the collar so that the inside diameter of the collar can be expanded and contracted by movement of the eccentric cam, which is connected to an external hex nut that can be turned to tighten the collar onto the mandrel or loosen the collar from the mandrel.
However, one drawback to this PCMC system is the steel-to-steel contact between the inside diameter of the collar and the outside diameter of the rotary mandrel. Whenever this bridge sleeve is slid onto the mandrel, there inevitably is some damage to the exterior surface of the mandrel by contact with the inside diameter of the collar. Moreover, due to the steel-to-steel contact between the inside diameter of the collar of each hub and the outside diameter of the mandrel, whenever there is a machine malfunction that results in a web wrap up event that prevents further advancement of the web being printed, the steel inside diameter of the collar will rotate with respect to the outside diameter of the mandrel. This metal-to-metal relative rotation mars the outside diameter of the mandrel by the involved steel-to-steel scraping. As much as a three inch circumferential scrape in the outside diameter of the mandrel can be anticipated by such events, requiring re-machining and repair of the mandrel at the expense of both the mandrel repair and the cost of the lost downtime of the printing machine.
Another disadvantage of this PCMC system is the fact that when the diameter of the bridge sleeve must be increased, a commensurate increase in the size of the hubs results in a significant increase in the weight of the bridge sleeve. Government workplace rules typically limit the weight of the bridge sleeve to no more than 50 pounds. Still another drawback to this PCMC system is the fact that the cam eventually starts to wear with use. Such wear then causes the collar to become loose and move with respect to the stabilizer. These movements cause the bridge sleeve to lose concentricity with the mandrel, which results in the bounce that causes deterioration of the print quality as described above. These unacceptable effects due to movement of the collar become more noticeable as the speed of rotation of the bridge sleeve increases and/or as the diameter and/or length of the bridge sleeve increases.